Array.prototype._map=function(fn,thisArg){
    let result=[];
    this.reduce((pre,curr,index,array)=>{
        result[index]=fn.call(thisArg,array[index],index,array);
    },0);
    return result;
}
const arr=[1,2,3,4];
let res=arr._map(item=>{
    return item+1;
})
console.log(res);